Powder coatings have been attracting a great deal of attention in recent years as eco-friendly paints, because powder coatings, which are solvent-free coatings, do not cause environmental pollution and are able to save resources. The scope of use of such powder coatings in place of solvent-based paints is expanding and their consumption is also on the steady increase.
Powder coatings have so far been used not only on automotive bodies, residential building materials, etc. but also in the field of road-related materials such as guardrails and road signs. However, none of the conventional powder coatings are capable of providing all the necessary properties for outdoor use such as corrosion resistance, weather resistance, chipping resistance, and the appearance of the coating film. For example, epoxy resin powder coatings offer corrosion resistance and chipping resistance sufficiently but are not fully satisfied in weather resistance. Polyester resin powder coatings and acrylic resin powder coatings are satisfactory in weather resistance but not satisfactory enough in the resistance to corrosion and chipping. Epoxy-polyester resin powder coatings fail to satisfy any of those properties.
Therefore, it has been investigated that formation of a multi-layer coating film by using two or more kinds of powder coatings which have different abilities improves these properties of film. In this connection, the forming of films from powder coatings is generally carried out in a procedure baking after each coating. For example, it is general that so-called 2-coat/2-bake method is adopted for formation of two layered film. However, since the multiple coating by this process takes much time, it is desired to develop a coating technology using the 2-coat/1-bake method, that is to say a coating process which comprises applying two coats successively and then curing both coats at the same time, for reduction of the processing time and conservation of resources.
However, in the 2-coat/1-bake system, the flowability in the bottom layer is inhibited by the top layer so that the coating particles, particularly coarse particles, in the bottom layer cannot flow well, thus giving rise to thin film spots in the top layer or the powders in the bottom layer migrate onto the surface of the top layer to detract from the final appearance of the coating film.
Japanese Kokai Publication Hei-6-304519 discloses a method of forming a multi-layer coating films which comprises applying a polyester resin powder coating or an acrylic resin powder coating on the uncured coating film from an epoxy resin powder coating and heat-curing the two-coats at the same time. In this method, the chipping resistance, corrosion resistance, and weather resistance of the final coating film can be improved by designing the coatings in such a manner that the surface tension of the powder coating for the top layer will be lower than that of the powder coating for the bottom layer. However, because the difference of curing speed between the top layer and the bottom layer causes strain and/or shrinkage of the coating film, this method failed to accomplish a sufficient improvement in appearance.
As a means for improving the appearance of a coating film on formation from powder coatings by the 2-coat/1-bake method, Japanese Kokai Publication Hei-6-256692 discloses a coating method which comprises defining the flowability in melting stage of the powder coatings forming the top and bottom layers, respectively, so that the flowability will be larger in the top layer. However, since the top layer takes a long time to be cured, this coating method gives no film which is cured completely within a practically acceptable curing time. So chipping resistance, weather resistance and corrosion resistance of this film are not good.